If leash is null it's not a valid state to remove PiP

Due to potential race condition if an app crashes during the dismiss
PiP gesture, it is possible that the leash will be reset in
onTaskVanished before the removePip function gets invoked. In such state it's no longer valid to try to dismiss PiP, since it's already gone.

Bug: 269113067
Test: n/a, following bug report stack trace for root cause

Change-Id: I985ee12f54d8fb2ab005197427a522d871692d1a